The present invention relates to a device for detecting an abnormality of an intake air measurement system for measuring an amount of induced air into an engine or a fuel injection system for injecting a fuel into a cylinder of the engine in an air-fuel control system mounted on the engine and a method therefor.
In order to promptly correct an irregularity of an intake air sensor and the like of an intake air measurement system or an injector and the like of a fuel injection system at the time of production or a deviation of an air-fuel ratio due to an aging change, it has been known that a learning control is employed for a feedback control with an air-fuel ratio sensor such as, for example, an O.sub.2 sensor or the like to always hold a desired air-fuel ratio even if an operating condition is largely varied.
More specifically, in a normal operating state of an engine, a center value of an air-fuel ratio feedback correction coefficient is stored in map of backup RAM of an electronic control system as learning value (a correction coefficient of an open loop) after the air-fuel ratio sensing O.sub.2 sensor repeatedly become rich and lean predetermined number of times by a proportional integration control and the air-fuel ratio feedback correction coefficient is calculated. Thereafter, if the operating condition varies, a fuel injection quantity is compensated by the learning value, a center of the air-fuel ratio feedback correction coefficient is controlled to become a standard value, and the air-fuel ratio of the air-fuel ratio control system of the engine is held at a desired air-fuel ratio.
In this case, in the air-fuel control system, a function for self-diagnosing an abnormality is incorporated so as to cope with a case where an abnormality occurs in a fuel injection system such as, for example, wirings of a fuel injection valve (injector) are disconnected or a short-circuited. For example, Japanese Patent Application Laid-Open 63-45443 discloses a technique for determining an abnormality of an air-fuel ratio controller including a fuel injection valve even when an air-fuel ratio feedback correction coefficient is limited to upper and lower limit values so that a learning value is not renewed, by judging whether or not an air-fuel ratio feedback control is executed when an engine is operated in each learning range for a predetermined time or longer and determining an abnormality of the air-fuel ratio controller when the air-fuel ratio feedback control is not executed in each learning range.
However, in the prior art as described above, only by detecting the abnormality in dependency on presence or absence of execution of the feedback control in a learning range, it is difficult to distinguish a change of the learning value due to an abnormality or deterioration of an intake air measurement system and/or a fuel injection system. And it is also difficult to distinguish the change due to the above mentioned abnormality or deterioration even if the learning value is changed in a predetermined range in a case where a feedback control is executed so that the learning is normally carried out.